1. Field of the Invention
The present invention relates to an optical information recording apparatus, optical information reproducing apparatus and an optical information recording/reproducing apparatus using holography, and specifically, to an optical information recording apparatus, an optical information reproducing apparatus and an optical information recording/reproducing apparatus in which an objective lens is moved for making access to a prescribed recording position or reproducing position of an optical information recording medium.
2. Description of the Related Art
In holographic recording for recording information onto a recording medium using holography, in general, the information light carrying image information and reference light are overlapped with each other in the recording medium and the interference pattern generated thereby is written onto the recording medium. For reproducing the recorded information, reference light is irradiated to the recording medium for diffracting the interference pattern so as to reproduce the image information.
The information light carrying the image information is generated by spatially modulating the intensity or the phase of the light. For example, Japanese Unexamined Patent Publication No. 11-311938 mentions holographic recording and modulation of light. In Japanese Unexamined Patent Publication No. 11-311938 (paragraph [0024]), it is noted that a liquid crystal device can be used as a spatial light modulator for spatially modulating light. Also, it is considered possible to use a DMD (digital micro mirror device) as a spatial light modulator for spatially modulating light. A DVD can modulate the intensity of incoming light by changing the reflection direction by each pixel and modulate the phase of the incoming light by changing the reflection position by each pixel.
However, when the pitch of pixels in the spatial light modulator is reduced for increasing the amount of information to be carried by the information light, a kind of diffraction grating is formed due to minute pixels of the spatial light modulator. Therefore, redundant diffraction light is also emitted from the spatial light modulator. If such diffraction light is also used as the information light, holography necessary for recording information becomes too large and the recording capacity of the recording medium is decreased. Thus, it is possible to prevent the holography from becoming too large by shielding the diffraction light.
However, when the diffraction light is shielded, the information light becomes dark by shielding light. Thus, efficiency of being able to use the information light becomes low.
As described, when the information is spatially modulated by minute pixels, it becomes necessary to deal with the diffraction light.
A prior application (Japanese Patent Application laid-open 2003-29968) discloses an optical information recording apparatus and optical information recording/reproducing apparatus in which diffraction light can be dealt with while suppressing a decrease in recording capacity of a recording medium and deterioration of efficiency for using the information light.
First, the optical information recording apparatus and the optical information reproducing apparatus of the prior application will be described.
FIG. 3 is an explanatory figure showing the configuration of the optical information recording/reproducing apparatus of the prior application. The optical information recording/reproducing apparatus includes an optical information recording apparatus and an optical information reproducing apparatus.
An optical information recording medium 1 used in the optical information recording/reproducing apparatus shown in FIG. 3 is formed by laminating a hologram recording layer 3 as an information recording layer to which information is recorded utilizing volume holography, a reflector film 5, a substrate (protective layer) 8 in this order on one face of a disk-type transparent substrate 2 formed with polycarbonate or the like.
The hologram recording layer 3 is formed with a hologram material in which the optical properties such as the refractive index, dielectric constant, reflectance and the like change according to the intensity of a laser beam when being irradiated by the laser beam for a prescribed length of time. For example, photopolymers HRF-600 (name of the product), the products of Dupont, or the like can be used as the hologram material.
The reflector film 5 is a film for reflecting light (reproduction-specific reference light and the like), and the reflector film 5 is formed with, for example aluminum.
The substrate (protective layer) 8 is a substrate with address formed by, for example, injection. In the substrate (protective layer) 8, an address servo area and a data area (not shown) are provided. By using the address servo area, irradiating position of light in the optical information recording medium 1 can be servo-controlled. Also, in the data area, information to be recorded in the optical information recording medium 1 can be recorded by means of holography.
A pickup 11 irradiates reference light and information light onto the optical information recording medium 1 and receives reproduction light from the optical information recording medium 1. The pickup 11 comprises an objective lens 12, an actuator 13, a quarter-wave plate 14, a half mirror 26, convex lenses 27a, 27b, an optical detector 28, a laser light source 32, a collimator lens 34, a half-wave plate 35, a polarization beam splitter 36, a mirror 38, a spatial light modulator (information expressing device) 40, a shutter 42, a convex lenses 44a, 44b, a diaphragm 46, a half-wave plate 48, a half mirror 50, a mirror 52, a mirror 54 and a convex lens 56.
The information light is the light to which information to be recorded is added. In FIG. 3, the information light is obtained by modulating the laser light generated by the laser light source 32 by the spatial light modulator 40 and is irradiated onto the optical information recording medium 1. The refraction light generated by the minute pixels of the spatial light modulator 40 also carries information, however, it is referred to as the refraction light as being discriminated from the information light in this specification.
As the reference light, there is recording-specific reference light for forming holography by being interfered with the information light and reproduction-specific reference light for reproducing information from the holography. Further, the reproduction light is the light returned from the optical information recording medium 1 to the pickup 11 when the reproduction-specific reference light enters the optical information recording medium 1. The reproduction light carries the information reproduced form the optical information recording medium 1.
The objective lens 12 is positioned on the transparent substrate 2 side of the optical information recording medium 1. The reference light and the information light enter the optical information recording medium 1 after transmitting through the objective lens 12. The reproduction light from the optical information recording medium 1 transmits through the objective lens 12 and proceeds towards the half mirror 26.
The actuator 13 is for moving the objective lens 12 in the thickness direction and the radial direction of the optical information recording medium 1.
The quarter-wave plate 14 converts the transmitted light from the linear polarization to circular polarization when the linear polarization light such as P-polarization or S-polarization enters and the angle of the direction of the linear polarization is 45° with respect to the optical axis of the crystal in the quarter-wave plate 14.
The information light is P-polarization and is converted to circular polarization after passing the quarter-wave plate 14, and enters the optical information recording medium 1. The reproduction light is circular polarization and is converted to S-polarization after passing the quarter-wave plate 14, and reaches the optical detector 28 through the convex lenses 27a, 27b. 
The half mirror 26 allows the information light to transmit and directs it to the quarter-wave plate 14, while reflecting the reproduction light and directs it to the convex lenses 27a, 27b. These convex lenses 27a, 27b, upon receiving the reproduction light, direct it to enter the optical detector 28. Further, the optical detector 28 receives and detects the reproduction light. Thereby, the information recorded in the optical information recording medium 1 can be reproduced.
The laser light source 32 is for generating the laser light. The laser light becomes the base for the information light and the reference light. The collimator lens 34 converts the laser light into parallel light rays upon receiving it from the laser light source 32. The half-wave plate 35 converts the parallel light rays from P-polarization to S-polarization upon receiving them from the collimator lens 34. The polarization beam splitter 36, upon receiving the P-polarization and the S-polarization from the half-wave-plate 35, allows the P-polarization to transmit through and reflects the S-polarization. The transmitted P-polarization proceeds towards the mirror 38 and the reflected S-polarization proceeds towards the convex lens 44a. The P-polarization becomes the base for the information light and the S-polarization becomes the base for the reference light. The mirror 38, upon receiving the P-polarization, reflects it towards the spatial light modulator 40.
The spatial light modulator (information expressing device) 40 generates the information light by reflecting the P-polarization upon receiving it from the mirror 38. The generated information light transmits through the shutter 42 and the polarization beam splitter 36, and proceeds towards the convex lens 44a. The diffraction light is also emitted from the spatial light modulator 40.
The shutter 42 is open when information is recorded in the optical information recording medium 1 and closed when information is reproduced from the optical information recording medium 1 (see FIG. 4). The convex lens 44a builds an image on the diaphragm 46 upon receiving the information light and the diffraction light from the spatial light modulator 40. At this time, the information light and the diffraction light near the optical axis passes through a hole 46a of the diaphragm 46. However, the diffraction light distant from the optical axis cannot pass through the diaphragm 46. The distance between the convex lens 44a and the diaphragm 46=distance between the convex lens 44b and the diaphragm 46=distance between the spatial light modulator 40 and the convex lens 44a=distance between the convex lens 44b and the mirror 52=f=focal distance. The half-wave plate 48 converts the S-polarization reflected by the polarization beam splitter 36 into the P-polarization. This becomes the reference light.
The half mirror 50 reflects the information light and directs it to the half mirror 26 while allowing the reference light to transmit and directs it towards the mirror 52. The mirror 52 reflects the reference light towards the mirror 54 which faces the optical information recording medium 1. The mirror 54 reflects the reference light and directs it towards the convex lens 56. The convex lens 56 diffracts the reference light so as to focus it onto a position in front of the optical information recording medium 1.
In the above-described configuration, when recording operation information about recording/reproduction of the optical information to the optical information recording medium 1, the laser light generated by the laser light source 32 transmits through the collimator lens 34, the half-wave plate 35, the polarization beam splitter 36, is reflected by the mirror 38, and proceeds to the spatial light modulator 40. Then, the laser light becomes the information light by the spatial light modulator 40, which then passes through the shutter 42, the polarization beam splitter 36, the convex lens 44a, the diaphragm 46, the convex lens 44b, the half-wave plate 48, is reflected by the half mirror 50, transmits through the quarter-wave plate 14, and proceeds towards the objective lens 12. The optical elements from the spatial light modulator 40 to the objective lens 12 (the convex lens 44a, the diaphragm 46, the convex lens 44b, the half-wave plate 48, the half mirror 50 and the like) correspond to an information light obtaining means for obtaining the information light from the spatial light modulator 40.
The laser light generated by the laser light source 32 becomes the recording-specific reference light after transmitting through the collimator lens 34, the half-wave plate 35 and then being reflected by the polarization beam splitter 36. The recording-specific reference light passes through the convex lens 44a, the diaphragm 46, the convex lens 44b, the half-wave plate 48, the half mirror 50, is reflected by the mirrors 52, 54, and is diffracted by the convex lens 56 to be heading towards the objective lens 12.
The objective lens (holography forming device) 12 forms holography through making the information light interfere with the recording-specific reference light in the hologram recording layer 3 of the optical information recording medium 1.
When reproducing information from the optical information recording medium 1, as shown in FIG. 4, the laser light generated by the laser light source 32 becomes the reproduction-specific reference light after transmitting through the collimator lens 34, the half-wave plate 35, and being reflected by the polarization beam splitter 36. The reproduction-specific reference light passes through the convex lens 44a, the diaphragm 46, the convex lens 44b, the half-wave plate 48, the half mirror 50, is reflected by the mirrors 52, 54 and is diffracted by the convex lens 56 to be heading towards the objective lens 12.
The objective lens (reproduction-specific reference light irradiating device) 12 diffracts the reproduction-specific reference light and irradiates it to the hologram recording layer 3. Thereby, the reproduction light carrying the information as the subject of reproduction is generated from the hologram recording layer 3 of the optical information recording medium 1. The reproduction light transmits through the objective lens 12, the quarter-wave plate 14, is reflected by the half mirror 26, transmits through the convex lenses 27a, 27b and then is received by the optical detector 28. The optical detector 28 receives and detects the reproduction light as the incoming light and obtains the information. The objective lens 12, the quarter-wave plate 14, the half mirror 26, and the convex lenses 27a, 27b correspond to a reproduction light obtaining means for obtaining the reproduction light. The optical detector 28 corresponds to an information reproducing means for reproducing information by receiving the reproduction light. The optical detector 28 is, for example, a CMOS sensor or a CCD array sensor.
Japanese Unexamined Patent Publication No. 11-311938 (paragraph [0024])
The above-described optical information recording/reproducing apparatus disclosed in the prior application has a configuration in which the pickup 11 moves for making access to a prescribed recording position and reproducing position of the optical information recording medium. However, the pickup 11 includes the objective lens 12, and also a number of optical devices, the light sources 32 and the like so that the pickup 11 becomes heavy. Thus, the driving device for driving the pickup becomes large-scaled and, as a result, the optical information recording/reproducing apparatus also becomes large-scaled.
Also, in the optical information recording/reproducing apparatus, the pickup 11 is heavy and, due to inertia, the pickup 11 cannot be brought to make access to the optical information recording medium at a high speed, thereby decreasing the transfer rate.
In order to overcome the foregoing problems, conventionally, in a CD (compact disc) drive or a DVD (digital versatile disc) as an optical information recording/reproducing apparatus which does not use holography, the objective lens is moved in accordance with the irradiation position of the recording medium since it is only required that the optical system transfers the intensity (energy) of the light.
However, the optical system of the holographic recording irradiates the spatially modulated information light and the recording-specific reference light onto the recoding medium 1 by the objective lens for making them interfere with each other in the information recording layer 3 of the recording medium 1 for achieving recording. Thus, it is necessary to build an image of at least the information light which is spatially modulated by the spatial light modulator (information expressing device) 40 in an incident pupil surface of the objective lens 12. Also, for reproduction, it is necessary to build an image of the reproduction light generated from the information recording layer 3 of the recording medium 1 by the reproduction-specific reference light in the optical detector 28 at last.
FIGS. 5A and 5B are illustrations showing the optical system of holographic recording in which the position of the spatial light modulator (information expressing device) 40 is fixed and the position of the objective lens 12 is moved.
As shown in FIG. 5A, when recording the optical information on the outer periphery side (left side in the figure) of the optical information recording medium 1 or reproducing the optical information from the outer periphery side of the optical information recording medium 1, it is necessary that the position of the spatial light modulator 40 shown as an image surface or the optical detector 28 and the incident pupil surface of the emission pupil surface of the objective lens 12 (positions of half mirrors 50: 26 in FIG. 5A) are set to be used alternately. For building an image on both positions, it is necessary that each space between the image surface and each of a pair of lenses 44a, 44b: 27a, 27b opposing to each other, the focal point f of the convex lens, the incident pupil surface: the emission pupil surface is set to be the first focal distance f1. The terms and numerals for reproduction are mentioned after “:“, following the terms and numerals for recording.
Meanwhile, as shown in FIG. 5B, when recording the optical information on the inner periphery side (right side in the figure) of the optical information recording medium 1 or reproducing the optical information from the inner periphery side of the optical information recording medium 1, it is necessary that each space between the image surface and each of a pair of lenses 44a, 44b: 27a, 27b opposing to each other, the focal point f of the convex lens, the incident pupil surface: the emission pupil surface is set to be the second focal distance f2. The movable range of the objective lens 12 and the half mirrors 50:26 is defined as D.
In order to build images in both the outer periphery side and the inner periphery side of the optical information recording medium as described by the image surface and the incident pupil surface or the emission pupil surface of the objective lens 12, the focal distance (thickness) of each of the convex lenses 44a, 44b: 27a, 27b has to be changed. Thus, the configuration in which only the objective lens is moved, which is employed in the conventional CD drive or the DVD drive, cannot be employed in holographic recording and the optical system from the image surface to the objective lens is moved as a whole.